1. Field of the Invention
The present invention relates to an exposure apparatus for production of integrated circuits (ICs) with an illuminometer for measuring intensity of illumination and/or distribution thereof on the water surface to be exposed by a light source.
2. Description of the Prior Art
The exposure apparatus for production of ICs of the type mentioned above is usually required to have high uniformity of illumination by which the wafer surface is to be exposed. In recent years, the degree of integration of ICs has become higher and higher up to the pattern line width of about 1 .mu.m. With the increase of the integration degree of ICs, the requirement for uniformity of illumination has become more and more severe. The control on the pattern line width is directly affected by irregularity of the illumination. Irregularity of illumination results in irregularity of line width.
Conventionally, the intensity of illumination of the exposure apparatus is measured by an illuminometer. According to the prior art, the illuminometer is disposed in a space between the illumination light projection part (for example, the light projection part of a projection lens system) and the sample stage on which a wafer is placed. Also, to measure the distribution of intensity of illumination with the illuminometer, the illuminometer is mounted on a mounting table having a mechanism for moving the illuminometer one-dimensionally or two-dimensionally. With this prior art arrangement, the intensity of illumination or the distribution thereof actually measured is only that existing between the illumination light projection part and the stage, not the intensity of illumination directly on the part to be actually illuminated, that is, the exposed surface of the wafer on which a pattern is to be printed. This is one of the disadvantages of the prior art exposure apparatus.
To meet the increasing integration degree of ICs, the exposure apparatus recently developed are generally complicated in structure. Because of the complicated structure, some of these known apparatus have no space available for mounting the illuminometer with or without the above mentioned moving mechanism between the light projection part and the stage. In such cases, a characteristic test is carried out beforehand on the illumination system at the time of manufacture of the apparatus to know the intensity of illumination or distribution thereof on the portion to be illuminated by the illumination system. After the preliminary test, all of the components are assembled together into an exposure apparatus for production of ICs. However, since such a preliminary measurement is carried out at a position deviated or completely different from the position of a wafer in the finally completed apparatus, the data of intensity of illumination obtained therefrom are mere relative ones. It is impossible to correctly know the real intensity of illumination or distribution thereof on the exposed surface of any wafer actually placed on the stage at any point in time. This constitutes a second disadvantage of the prior art apparatus.
Furthermore, there is another problem in the prior art apparatus. The output of the lamp at the illumination light projection part decreases gradually with time. Conventionally, the deterioration of the lamp has been judged on the basis of only the lit time thereof. When the lamp has been used for illumination for a certain predetermined time, one judges this to be the end of the life of the lamp. In other words, replacement of the old lamp has conventionally been done based on a mere rough estimate of the useful life of the lamp. Obviously, this method is subject to errors.